In producing photographic film or paper, it is necessary to coat the film support or paper with discrete layers of photographic coatings. Some of these layers contain a radiation sensitive material like silver halides, zinc oxide, titanium dioxide, diazonium salts, and light sensitive dyes as well as other photographic additives including matting agents, developing agents, mordants, etc. Other layers may contain materials which are not radiation sensitive like subbing layers, pelloid protective layers, filter layers, antihalation layers, and interlayers. Additionally, hydrophilic colloids, polysaccharides, surfactants and synthetic polymers may also be incorporated in photographic coating liquids.
The number of separate and discrete layers of photographic coatings applied to photographic paper or film support depends on the product's design. Typically, the number of layers varies between 1 to 15, more usually 3 to 13.
A multi-slide hopper is a known apparatus which will simultaneously coat two or more liquids onto a solid support in such a way that the layers are not mixed and are individually of uniform thickness. The conventional slide hopper performs its coating operation by metering a first coating liquid from a supply through a narrow slot which distributes the liquid uniformly across the top of a downwardly inclined slide surface. This layer of liquid moves down the slide surface by gravity to supply a steady, uniform, smooth coating layer to a coating bead across which it is applied to the web being coated. A second coating liquid is supplied to and distributed by, a second slot which directs a uniform layer of that liquid onto the top of a second slide surface. The second coating liquid first flows down its own slide surface and then onto the top of the layer of liquid issuing from the first slot. The layers of the first and the second liquids then together flow down to a coating bead where they are applied to the web. Additional liquids may be coated simultaneously by equipping the hopper with the appropriate number of slots and slide surfaces.
Instead of applying photographic coatings from a multi-slide hopper to a web by use of a coating bead, multi-layer photographic coatings can be applied by passing the web beneath a liquid curtain formed by discharging the coating liquid from a terminal lip portion of the multi-slide hopper. Both the bead coating and curtain coating techniques are well known, as disclosed e.g., in U.S. Pat. No. 4,287,240 to O'Connor.
Photographic liquids are generally pumped from a supply to a slot at the hopper's slide surface through passages in the coating hopper. To dampen flow surges and achieve thickness uniformity in the applied coatings, the passages include one or more transverse distribution channels. Such distribution channels receive photographic liquid from a relatively narrow feed conduit and spread it transversely so that it forms a liquid layer distributed across the bopper width when discharged from the slot. Distribution occurs due to the hopper's low resistance to transverse liquid flow and its high resistance to longitudinal flow toward the slot. These distribution channels have been formed with a variety of cross-sectional configurations, including circular shapes (see, e.g.. U.S. Pat. No. 4,041,897 to Ade), semi-circular shapes (see, e.g.. U.S. Pat. No. 4,109,611 to Fahrni et al.), and triangular shapes (see, e.g., U.S. Pat. No. 3,005,440 to Padday). Generally, such configurations have the same cross-sectional shape at all locations across the hopper. However, distribution channels can also be designed to narrow as they extend transversely outward within the hopper (see e.g. Swiss Patent No. 530,032 to Ciba-Geigy AG).
When a single distribution channel is utilized, product non-uniformities can occur due to imperfect channel fabrication as well as deviations from flow rates, viscosities, temperatures, and pressures of the coating liquid for which the channel was designed. To counteract these problems, it has been found advantageous to place a secondary distribution channel in the photographic liquid passages of the hopper downstream of the primary distribution channel. Like the primary distribution channel, the secondary distribution channel is configured to impose a low resistance to transverse liquid flow and a high resistance to longitudinal liquid flow toward the slot exit. As a result, any transverse pressure non-uniformities in liquid emerging from the primary distribution channel are substantially reduced. See Swiss Patent No. 530,032 to Ciba-Geigy AG, British Patent No. 1,389,074 to GAF Corporation, and K. Lee and T. Liu, "Design and Analysis of a Dual-Cavity Coat Hanger Die," Polymer Engineering and Science. vol. 29, no. 15 (mid-August 1989), which discloses the use of two distribution channels generally.
In polymer extrusion, where secondary distribution channels have also been utilized, the cross-sectional shape of that channel is not critical due to the narrow range of solution properties and process conditions encountered. These properties and conditions are generally defined in terms of a Reynolds Number which is defined as follows: ##EQU1## where: .rho. is the fluid density
.mu. is the fluid viscosity PA1 q is the flow rate per unit width (i.e. the flow rate at the secondary distribution channel inlet divided by width of the hopper perpendicular to the channel cross-section).
For polymer extrusion, the Reynolds Number is generally about zero because of very high fluid viscosity. With such a low Reynolds Number, the primary function of the secondary distribution channel becomes merely the reduction of non-uniformity in fluid distribution resulting from imperfect hopper manufacture. However, when moderate Newtonian viscosity and/or high flow rates are encountered, as in the coating of photographic materials, such non-uniformity is more likely to occur due to variations in fluid parameters rather than imperfect hopper design. To ameliorate such non-uniformity, the cross-sectional area of the secondary distribution channel should be increased. This creates additional problems, however, including the onset of flow recirculation (i.e. eddying) within the secondary distribution channel, and sedimentation of solids in the liquid.
FIGS. 2A to D show fluid flow in a side cross-sectional view of a secondary distribution channel with a commonly-used semi-circular shape at Reynolds Numbers of 0, 10, 12, and 20, respectively. This configuration is semi-circular in that the center of the circle lies in the plane of slot-forming wall 200 of hopper plate 202. In each of these figures, fluid traveling along the path defined by arrow F enters the channel and travels along the depicted paths. As the Reynolds Number is increased from a very low value (i.e. Re=0) to Re=20, we see smooth flow for FIG. 2A, the onset of separation from channel wall 204 at the entrance to the channel in FIG. 2B, a developed eddy in FIG. 2C, and, finally, a full eddy encompassing a large portion of the channel in FIG. 2D. It is thus apparent that in prior art designs of secondary distribution channels a substantial growth in the size of an eddy takes place as the Reynolds Number increases.
For photographic coatings, it is believed that eddies in the secondary distribution channel may entrap foreign materials in the coating solution during purge flow conditions (i.e., at high Reynolds Numbers used to remove flush water and/or air from the channel). These materials may then be released into the flow stream at coating conditions (i.e., at lower Reynolds Numbers) and may re-lodge on the walls of the hopper downstream of the eddying region (e.g., at the slot for that liquid, on the slide, or on the coating lip). This can generate streaks in the product which is unacceptable for high quality products. As a result, the hopper must be periodically shut down and purged to remove particles. This procedure increases waste and diminishes product output.
Eddies in the flow field during coating are also known to increase dramatically the residence time of that portion of the solution caught in the recirculating zone. In photographic liquids with time dependent chemical reactions, this may cause the resulting product to have a more non-uniform composition which does not meet specifications.
In recognition of these problems, hopper designers have taken a number of approaches to eliminate or reduce the presence of eddies in the flow field. For example, the configuration of the secondary distribution channel has been changed from a semi-circular shape to a shorter circular segment. FIGS. 3A to D show fluid flow in a side cross-sectional view of a secondary distribution channel with a circular segment shape at Reynolds Numbers of 0, 15, 18, and 20, respectively. This segment is less than 180.degree. so that the center of a full circle containing this segment lies within hopper plate 302 somewhat distal from slot-forming wall 300. These drawings show no eddy at a Reynolds Number of 0 (FIG. 3A). As the Reynolds Number is increased to 15, a minor eddy develops (FIG. 3B). Major eddying and a yet larger eddy appear at Reynolds Numbers of 18 and 20, respectively, as shown in FIGS. 3C and 3D, respectively.
A comparison of FIGS. 3A to D with FIGS. 2A to D shows that the onset of flow recirculation is postponed to a higher Reynolds number with the circular segment configuration of FIGS. 3A to D. However, the use of a circular segment configuration achieves only a modest delay of eddying and reduces the cross-sectional area of the secondary distribution channel, which, in turn, diminishes its ability to reduce non-uniformities. As a result, the need for a properly configured secondary distribution channel continues to exist.